Many construction machines, transportation devices, and other equipment employ an engine as a primary power source, with a number of other systems being driven off of the engine output. For example, in certain construction machines, a number of machine functions and operations are executed via hydraulic power. In such machines, a hydraulic pump is keyed to and driven by the engine output to supply hydraulic power to the machine components.
However, the hydraulic requirements of a machine are not necessarily directly related to the engine speed. For example, during a period of high engine speed, there may be little or no hydraulic power required under certain circumstances. Conversely, during periods of low engine speed, circumstances may dictate that a high level of hydraulic power is required. Thus, there is a certain intractable inefficiency resulting from the direct drive of the hydraulic pump from the engine output. If a smaller and more efficient pump is fitted to the engine, the pump may be unable to supply the machine's maximum hydraulic needs. However, if a larger less efficient pump is fitted to the engine, then while the pump will always be able to supply the required hydraulic flow, it will be underutilized during all but the most extreme operations.
Moreover, certain machines operate in a hybrid manner rather than relying exclusively on engine power. That is, such machines are capable of operating exclusively on electrical power, exclusively on engine power, or on a combination of the two. For example, both drive types may be used together when substantial acceleration is required, but the engine may be shut down or idled in favor of the electric drive mode of operation during machine launch conditions and low-speed cruise conditions. In such hybrid machines, the exclusive use of a single direct drive hydraulic pump will mean that hydraulic power is completely unavailable during electric drive.
Thus, it will be appreciated from this disclosure that the traditional engine-driven hydraulic pump causes issues of inefficiency (or lack of capacity) in many machines, and also results in a loss of hydraulic flow during electric drive operating modes in hybrid machines.
Although the resolution of deficiencies, noted or otherwise, of the prior art has been found by the inventors to be desirable, such resolution is not a critical or essential limitation of the disclosed principles. Moreover, this background section is presented as a convenience to the reader who may not be of skill in this art. However, it will be appreciated that this section is too brief to attempt to accurately and completely survey the prior art. The preceding background description is thus a simplified and anecdotal narrative and is not intended to replace printed references in the art. To the extent an inconsistency or omission between the demonstrated state of the printed art and the foregoing narrative exists, the foregoing narrative is not intended to cure such inconsistency or omission. Rather, applicants would defer to the demonstrated state of the printed art.